1. Field of the Invention
The present invention relates to an aluminum alloy sheet suitable for use as an automobile body sheet and for making formed parts of household electric apparatuses, and a method of producing the same. More specifically, the present invention provides an aluminum alloy sheet having excellent strength, formability and weldability at low cost.
2. Description of the Related Art
As a result of the recent demand for a reduction in weight of automobile bodies, extensive use of aluminum alloy sheets for body sheets is being considered. Accordingly, aluminum alloy sheets are required to be excellent in press formability, weldability and strength as conventional cold-rolled steel sheets. To meet such requirements, 5000-Series alloys of the Al--Mg type and, more specifically, Alloys No. 5052, 5182, etc. are being employed. A problem with these alloys, however, is that their r-values, which serve as an index of ductility and deep drawability, are much lower than those of steel sheets. Thus, it is difficult for these alloys to be worked in a manner equivalent to steel sheets, so that their application is restricted to parts not requiring much working, such as hoods.
Further, aluminum alloy sheets are poorer in resistance-spot-welding properties as compared with steel sheets. In particular, they have a problem in that electrode life during continuous spot welding tends to be extremely short, so that dressing prior to electrode life expiration or electrode replacement has to be frequently performed, resulting in poor production efficiency.
Various efforts have been made to attain an improvement in the formability of aluminum alloy sheets. For example, as disclosed in Japanese Patent Laid-Open No. 61-130452, a method has been developed according to which an improvement in elongation is attained by setting an upper limit to the amounts of Fe and Si and, at the same time, adding a large amount of Mg. With these techniques, it has been essential, from the viewpoint of formability, to use a new raw metal (a new aluminum ingot, a prime metal) having a high purity of 99.7% or more, in both conventional 5000-Series metals and newly developed high-ductility alloys, as the raw metal thereof, due to the restriction in purity to ensure the requisite elongation.
However, as is well known, new aluminum raw metal is expensive, so that aluminum alloy sheets are much more expensive than steel sheets.
Nevertheless, the elongation percentage of aluminum sheets obtained by the above-described conventional techniques is not more than 40%, which is markedly lower as compared with 40% or more of steel sheets.
As disclosed in Japanese Patent Laid-Open No. 4-123879, a method has been developed of providing an electrically insulating coating on the surface of an aluminum alloy sheet in order to achieve an improvement in weldability (evaluated by the length of electrode life), which method, however, does not help to improve formability and weldability.